This invention relates to a valve which directs fluid flow from an inlet port to a pair of outlet ports. The invention relates particularly to a priority valve which divides fluid flow from a pump between a vehicle steering circuit and an auxiliary fluid power circuit.
A known priority valve for dividing fluid flow between a vehicle steering circuit and an auxiliary circuit is shown in U.S. application Ser. No. 361,851, filed Mar. 25, 1982, and is assigned to the assignee of this invention. The known priority valve includes a movable valve spool which controls fluid flow to the vehicle steering circuit and the auxiliary fluid power circuit. The valve spool is spring biased to a priority position in which it directs all fluid from a fluid source to the steering circuit. The valve spool can move away from its priority position when a fluid pressure differential applied across the valve spool exceeds the spring bias thereon.
The fluid pressure differential on the valve spool is produced by fluid pressures in a pilot fluid circuit that branches from the priority flow communicated to the steering circuit. Fluid pressure in the pilot circuit is controlled by a hydrostatic steering controller in the steering circuit, and can vary in accordance with the steering demand (i.e. the rate at which an operator steers and the resistance encountered by the vehicle's wheels). When the fluid flow and pressure directed to the steering circuit are sufficient to effect steering as demanded by an operator, the fluid pressure in the pilot circuit causes the priority valve spool to shift away from its priority position, and direct fluid that is not needed for steering to the auxiliary circuit where it is available for operating one or more auxiliary fluid power implements (e.g. back hoe, end loader) carried by the vehicle. When flow and pressure in the steering circuit are insufficient to effect steering as demanded by an operator, the priority valve spool is rapidly returned toward its priority position, so that more fluid is made available for steering until the steering demand is satisfied.
In the system of U.S. patent application Ser. No. 361,851, the priority valve spool moves axially in a valve housing. The valve spool has lands that cooperate with surfaces of the housing to define variable size orifices for directing fluid to the steering and auxiliary circuits. The fluid flow to the steering and auxiliary circuits is across the lands that form the variable size orifices.
Applicant has found that with a valve such as disclosed in Ser. No. 361,851, when there is a high rate of flow across the lands of the valve spool to the priority port there are forces induced on the valve spool. The forces tend to urge the valve spool in a direction that restricts the priority flow. Such a condition can interfere with proper operation of a priority valve when the flow rate across the lands of the valve spool to the priority port is high and the auxiliary circuit is operating at a pressure that is significantly higher than the pressure in the steering circuit. The valve spool should assume and maintain its priority position to satisfy the steering demand, but the forces induced on the valve spool because of the increase in the pressure drop and the high rate of flow across those lands tend to urge it in a direction that restricts the flow to the priority port. Thus, the valve spool may not deliver the appropriate amount of priority flow to the steering circuit.